How to increase the resolution of a rendered image?
Given that the question is about resolution.
See that slider that shows 35% in the resolution settings?
You are rendering an image that is rougly a third of the resolution. Instead of 1200 pixels you are rendering only 420.
Set the value to 100%

As for why the picture is blurry, there is no  information on the question that can yield a conclusive answer. The focus or depth of field of the camera might be set incorrectly, or there might be some nodes in the compositor we don't know about.